Vent size for toilet, shower, and vanity for IPC

[Mayo]

Hey everyone,

I have a bathroom group that consists of a toilet, shower, and vanity.

The drain for the vanity is 2 inches and serves as a wet vent for the shower and the toilet. This is all pretty standard, but the tricky part is when the wet vent converts to a dry vent, I'd like to down size to 1.5 inches. This is because I have a 2x4 wet wall and I can't bore 2.5 inch holes (due to building code) to fit a 2 inch dry vent.

According to UPC a toilet must vent off a 2 inch pipe. However, I'm in Michigan and it follows the 2015 Michigan plumbing code which is the 2015 IPC. In the IPC it shows that any pipe must vent off a minimum half of its size. So my 3 inch toilet pipe has to have a 1.5 inch vent. According to the chart a 1.5 inch vent from a 3 inch pipe can service 10 dfu, a toilet, vanity, and shower add up to 6.

Therefore, I'm concluding that after the wet vent portion, I can convert to a 1.5 inch vent.

Am I missing something here? I don't see any hidden clause that says otherwise.

 

[John Gayewski]

I guess the wording in the section on wet venting would come into play here. Is there a portion of the wording that says "undiminished in size"? As in a wet vent shall be minimum 2" and blah blah blah... shall terminate undiminished in size blah blah blah...

@wwhitney would be the expert on these finer points in ipc.

 

[Sponsor]

 

[wwhitney]

Under the IPC, a 1.5" dry vent at the lavatory suffices for wet venting a bathroom group. The portion of the wet vent carrying only 1 DFU (i.e. a single lavatory) can even be 1.5", but 2" is good.

Cheers, Wayne